This invention relates generally to the field of emissions control for internal combustion engines, and more particularly to a positive crankcase ventilation valve (PCV valve) for reducing blow-by gases.
In a four-stroke internal combustion engine, lubrication is provided to the pistons within the cylinders by way of the crankcase. The piston rings serve to seal the cylinder chamber from the crankcase as the piston travels up and down. Of course, a perfect seal can never be obtained. As a result, oil from the crankcase intended for use in lubricating the cylinder gets drawn into the combustion chamber and burned during the combustion process. Also, combustion waste gases can escape past the piston rings into the crankcase. The phenomena of waste gases entering into the crankcase is commonly referred to as "blow-by". Because of blow-by, the concentration of hydrocarbon waste gases in the crankcase is often several times that of the concentration of hydrocarbons in the intake manifold. The presence of hydrocarbon gases in the crankcase presents a dilemma: if the gases are freely discharged into the atmosphere, the hydrocarbon waste gases contribute to urban pollution; however, if the waste gases remain trapped in the crankcase, the gases contaminate the lubricating oil and result in degraded oil performance, i.e. the contaminated oil does not lubricate as well as it should, contributing to unnecessary wear of the piston rings which subsequently reduces their ability to seal properly. Thus, it can be seen that a vicious cycle is established: imperfections in the piston rings that allow engine oil to be burned in combustion also allow blow-by gases to enter the crankcase and contaminate the oil; this contributes to degraded oil lubricating performance which increases piston ring wear, thus increasing the amount of oil allowed to burn in the combustion process and increasing the amount of blow-by gases that enter the crankcase, which further contaminates the oil, etc. Yet, because of concerns over urban pollution, simply venting the blow-by gases from the crankcase directly to the atmosphere is unacceptable.
To solve this problem, valves have been introduced for recirculating the blow-by gases from the crankcase into the intake manifold where the gases are re-burned during combustion. These valves, known as positive crankcase ventilation (PCV) valves, rely on differentials in pressure between the crankcase and the intake manifold to draw the blow-by gases from the crankcase into the intake manifold.
Because air is drawn into the cylinder combustion chamber through the intake valve as the piston travels downward, the air pressure within the intake manifold is lower than that of the surrounding ambient atmosphere. This is commonly referred to as "engine vacuum". The amount of vacuum varies according to the load upon the engine. When the engine is running at essentially a constant speed, a higher degree of vacuum is present than when the engine is under a loaded condition, such as when accelerating or climbing a hill.
Since the purpose of the PCV valve is to vent blow-by gases from the crankcase into the intake manifold, it can be appreciated that the reciprocal of this goal is that no gases from the intake manifold be allowed to enter the crankcase. Thus, most PCV valves known to date operate similar to a check valve: allowing blow-by gases to be drawn from the crankcase into the intake manifold while preventing gases from the intake manifold to be drawn into the crankcase. One common way to implement such a check valve is to provide for a normally closed spring biased valve. At rest, a spring holds an valve member, such as a ball or disk, over an orifice thereby occluding the flow of gases through the orifice. When the air pressure is greater on the valve member than the spring force, the spring is compressed and the valve member is unseated from the orifice, allowing gases to flow.
While it is ultimately the differential in pressure between the intake manifold and the crankcase that allows blow-by gases to be drawn from the crankcase through the PCV valve and into the intake manifold, the fact that engine vacuum varies as a function of engine load rather than engine speed means that blow-by gases will not always be properly vented when using a simple check valve type design. For example, as was discussed earlier, engine vacuum is higher when engine speed remains relatively constant. Thus, there is essentially the same amount of vacuum present when the engine is idling (approximately 800 rpm) as when the engine is at a highway cruising speed (approximately 2400 to 3000 rpm). However, it can be appreciated that greater amounts of waste gases are being generated at 2400 rpm than at 800 rpm. Similarly, when the engine is being loaded as occurs during acceleration or hill climbing, engine vacuum decreases and waste gases increase. Thus, relying solely on intake vacuum to vent crankcase gases, a choice must be made between providing too much ventilation at engine idle or providing too little ventilation at higher engine speeds or during loaded engine conditions and thereby risking oil contamination by the blow-by gases.